I understand ( to a point ) PPP protocol. With it we can establish
direct connection between two nodes ( I assume the nodes are connected
with analog line )

Now why do we need special protocol to handle direct connections?

1)
Say for example, that we would like to include certain PC into
Ethernet LAN network as one of LAN's nodes.

Only problem is that this PC is too far away and thus only way it can
connect to this LAN is via telephone cable.

* Why can't LAN treat this telephone cable as just another one of its
cables in its network ( I imagine an adapter of some kind would be
needed ) and thus packets sent through the telephone wire would be
usual Ethernet packets?

* Why needs the ethernet datagram ( or any type of LAN datagram ) be
encapsulated inside PPP? Does it have something to do with telephone
lines being analog or...?

2)
Is PPP needed only when direct links are analog?

thank you

10-02-2007, 09:53 PM

unix

Re: I kinda understand how PPP works, but...

In article <1181514709.214134.189500@q69g2000hsb.googlegroups.com>,
[email]kaja_love160@yahoo.com[/email] wrote:
[color=blue]
> hello
>
> I understand ( to a point ) PPP protocol. With it we can establish
> direct connection between two nodes ( I assume the nodes are connected
> with analog line )
>
> Now why do we need special protocol to handle direct connections?[/color]

Network and Transport layer protocols generally expect to use a Link
Layer protocol that provides a number of functions such as framing and
network layer identification.
[color=blue]
>
>
> 1)
> Say for example, that we would like to include certain PC into
> Ethernet LAN network as one of LAN's nodes.
>
> Only problem is that this PC is too far away and thus only way it can
> connect to this LAN is via telephone cable.
>
> * Why can't LAN treat this telephone cable as just another one of its
> cables in its network ( I imagine an adapter of some kind would be
> needed ) and thus packets sent through the telephone wire would be
> usual Ethernet packets?[/color]

Ethernet is a multi-access medium, so it has requirements that a
point-to-point link doesn't. Why would you want to waste 24 bytes on
addressing when a point-to-point link just connects two devices directly?
[color=blue]
>
> * Why needs the ethernet datagram ( or any type of LAN datagram ) be
> encapsulated inside PPP? Does it have something to do with telephone
> lines being analog or...?[/color]

Ethernet datagrams aren't encapsulated inside PPP. IP datagrams are.
Just as on an Ethernet, IP datagrams are encapsulated inside Ethernet
frames. Basically, for every medium, you need a way of encapsulating IP
datagrams into that medium's framing mechanism.
[color=blue]
>
> 2)
> Is PPP needed only when direct links are analog?[/color]

PPP is needed on point-to-point links that don't have their own framing
and addressing mechanism. This usually means serial links, because
modems just deal with bytes, not frames.

On Jun 11, 1:59 am, Barry Margolin <bar...@alum.mit.edu> wrote:[color=blue]
> In article <1181514709.214134.189...@q69g2000hsb.googlegroups.com>,
>
> kaja_love...@yahoo.com wrote:[color=green]
> > hello[/color]
>[color=green]
> > I understand ( to a point ) PPP protocol. With it we can establish
> > direct connection between two nodes ( I assume the nodes are connected
> > with analog line )[/color]
>[color=green]
> > Now why do we need special protocol to handle direct connections?[/color]
>
> Network and Transport layer protocols generally expect to use a Link
> Layer protocol that provides a number of functions such as framing and
> network layer identification.
>
>
>[color=green]
> > 1)
> > Say for example, that we would like to include certain PC into
> > Ethernet LAN network as one of LAN's nodes.[/color]
>[color=green]
> > Only problem is that this PC is too far away and thus only way it can
> > connect to this LAN is via telephone cable.[/color]
>[color=green]
> > * Why can't LAN treat this telephone cable as just another one of its
> > cables in its network ( I imagine an adapter of some kind would be
> > needed ) and thus packets sent through the telephone wire would be
> > usual Ethernet packets?[/color]
>
> Ethernet is a multi-access medium, so it has requirements that a
> point-to-point link doesn't. Why would you want to waste 24 bytes on
> addressing when a point-to-point link just connects two devices directly?
>
>
>[color=green]
> > * Why needs the ethernet datagram ( or any type of LAN datagram ) be
> > encapsulated inside PPP? Does it have something to do with telephone
> > lines being analog or...?[/color]
>
> Ethernet datagrams aren't encapsulated inside PPP. IP datagrams are.
> Just as on an Ethernet, IP datagrams are encapsulated inside Ethernet
> frames. Basically, for every medium, you need a way of encapsulating IP
> datagrams into that medium's framing mechanism.
>
>
>[color=green]
> > 2)
> > Is PPP needed only when direct links are analog?[/color]
>
> PPP is needed on point-to-point links that don't have their own framing
> and addressing mechanism. This usually means serial links, because
> modems just deal with bytes, not frames.
>
> --
> Barry Margolin, bar...@alum.mit.edu
> Arlington, MA
> *** PLEASE post questions in newsgroups, not directly to me ***
> *** PLEASE don't copy me on replies, I'll read them in the group ***[/color]

It makes sense now

thank you

10-02-2007, 09:53 PM

unix

Re: I kinda understand how PPP works, but...

Barry Margolin <barmar@alum.mit.edu> writes:[color=blue]
> In article <1181514709.214134.189500@q69g2000hsb.googlegroups.com>,
> [email]kaja_love160@yahoo.com[/email] wrote:[color=green]
> > * Why can't LAN treat this telephone cable as just another one of its
> > cables in its network ( I imagine an adapter of some kind would be
> > needed ) and thus packets sent through the telephone wire would be
> > usual Ethernet packets?[/color]
>
> Ethernet is a multi-access medium, so it has requirements that a
> point-to-point link doesn't. Why would you want to waste 24 bytes on
> addressing when a point-to-point link just connects two devices directly?[/color]

In addition, Ethernet protocol itself requires a set of bitwise
signaling that would be either impossible or just impractical on
modems. With a typical modem doing V.42, how would anyone create
Ethernet-like frames? What would an end of frame delimiter look like?
(How do you know it's done?)
[color=blue][color=green]
> > 2)
> > Is PPP needed only when direct links are analog?[/color]
>
> PPP is needed on point-to-point links that don't have their own framing
> and addressing mechanism. This usually means serial links, because
> modems just deal with bytes, not frames.[/color]

But certainly not "only," nor does it have anything to do with
"analog" (whatever that might mean in this context -- perhaps "async"
was wanted).

PPP is used, as the name implies, for point-to-point links. Besides
modems and async serial links, this also includes synchronous links
such as ISDN B channels, T1 lines, SONET, and a range of others.

Has the original poster read RFC 1661? If not, then I'd suggest doing
so. It explains the operation of PPP. (RFC 1547 might also be
helpful for some historical context.)

One bit of possible confusion here is that modern Ethernet also uses
point-to-point links, in the form of twisted-pair wires. However, the
behavior of the signaling on those wires is quite different from the
sorts of links that PPP uses. They behave instead as a shared medium.

[email]kaja_love160@yahoo.com[/email] writes:
[color=blue]
> * Why can't LAN treat this telephone cable as just another one of its
> cables in its network ( I imagine an adapter of some kind would be
> needed ) and thus packets sent through the telephone wire would be
> usual Ethernet packets?[/color]

Well, HomePNA is this. Ethernet over telephone cable.

I have some time used HomePNA-Ethernet adapter. It is just
as you described :-) Look from your nearest market.

/ Kari Hurtta

10-02-2007, 09:53 PM

unix

Re: I kinda understand how PPP works, but...

Kari Hurtta <hurtta@localhost.localdomain> writes:
[color=blue]
> [email]kaja_love160@yahoo.com[/email] writes:
>[color=green]
> > * Why can't LAN treat this telephone cable as just another one of its
> > cables in its network ( I imagine an adapter of some kind would be
> > needed ) and thus packets sent through the telephone wire would be
> > usual Ethernet packets?[/color]
>
> Well, HomePNA is this. Ethernet over telephone cable.[/color]

Not exactly. HPNA does *not* run Ethernet framing over a telephone
line, and is certainly not end-to-end over a telephone call (like PPP
is). It's more like DSL than like anything else.